1. Field of the Invention
This invention relates to a high-voltage direct-current supply system with over-current protection, including a high-voltage source having an output voltage, a current-measuring device for measuring a load current which flows through a load connected to the high-voltage source, a switching device for rapidly switching the load current, and a limit detector which is connected to the current-measuring device and drives the switching device when a predetermined limit of the load current is exceeded, via a switch control unit in such a manner that the load current is interrupted.
Such a supply system, as above briefly described on which high demands are made with regard to capacity and especially short-circuit safety, is known, for example, from the field of fusion research (P. L. Mondino and K. I. Selin in: Proc. of the 7th Symposium on Engineering Problems of Fusion Research, Knoxville, U.S.A., Oct. 25-28, 1977, p. 1558-1583), where it is provided as a supply unit for the additional heating of the plasma in a fusion reactor.
Depending on the method of plasma heating used, direct voltages within the range of 30-200 kV are needed at currents within the order of magnitude of 50 A. If the heating is carried out in particular by injecting a beam of neutral particles, short-circuits can repeatedly occur in the requisite injector system needed during the start-up phase and also during the beam injection proper, lasting several seconds, as a result of flashovers in the electrode system of the beam gun. In order to prevent damage to the sensitive injector system by such short-circuits, there is needed a rapidly responding over-current protection which reliably interrupts the current flow within less than 10 .mu.s and switches the high voltage on again after a recovery time of 1-10 ms.
According to the aforementioned prior art, the high-voltage source is connected, for this purpose, via the series-connected switching device to the load, that is to say the injector system. The load current flows through the closed switching device and the current-measuring device arranged in an output line where its intensity is continuously measured during operation. But as soon as the load current exceeds a predetermined limit, the connected limit detector supplies a control signal to the switch control system which, in turn, opens the switching device and thus interrupts the flow of the load current.
Since the switching device is arranged in the output line of the supply system, it must not only be able to switch the load currents occurring but, above all, it must have an extraordinarily high electric strength so that the breaking process is not endangered. For the reasons mentioned, in the prior art a high-power tetrode is used as the switching device. The tetrode carries the load current as anode current when in the conducting state and in the short-circuit case breaks the load current by means of suitable control voltages being applied to its control grids.
However, the use of a high-power tetrode as a switching device between the high-voltage source and the load presents various problems. As a result of the cathode heater needed for operation and the high voltage drop of some kV between cathode and anode when in the conducting state, the tetrode has an internal power consumption of several 100 kW, which not only degrades the efficiency of the supply but must also be dissipated by appropriate cooling measures.
Similarly, with the currents and voltages occurring during switching operation, the electrodes of the tube are subjected to considerable stresses which can lead to a deformation of the internal tetrode structure and thus to a failure of the switching device.
Finally, in the case of a short-circuit, the breaking process is carried out by only one switching element, the tetrode, so that for the case of a malfunction of this tetrode a crowbar circuit connected in parallel with the source and requiring additional expenditure on components and control circuits must also be provided between the switching device and the high-voltage source.